Go beyond Earth and deeper into the solar system, past the craggy terrain of Mars and the shape-shifting storm of Jupiter, through the delicate rings of Saturn, beyond the silky clouds of Uranus and Neptune, and you will find a mysterious zone of small, icy objects. They number in the millions, some half the size of the continental United States, others as small as cities. They form a ring around the solar system, silent sentries guarding the blazing sun, which is so distant that it looks like any other star would.

Back in the day, about 4.6 billion years ago, the solar system was little more than a cloud of cosmic dust spinning around a newborn star. Gravitational forces pulled and smoothed some of the dust into spheres—the planets and moons. The small bodies here, in a region known as the Kuiper Belt, were left out in the cold, and have remained virtually unchanged all this time.

This year, NASA stopped by. The New Horizons spacecraft flew past one of these objects, snapping pictures and collecting scientific data as it went. No space mission had ever visited a target so far away, and NASA even held a New Year’s Eve party for scientists and engineers to count down to the historic pass around midnight.